The present invention relates to an image forming apparatus having an image carrier and a developing device for developing an electrostatic latent image formed on the image carrier to produce a corresponding toner image.
With an electronic copier, facsimile transceiver, printer or similar image forming apparatus of the type using an image carrier, e.g., a photoconductive element, it is a common practice to electrostatically form a latent image on the image carrier by, for example, exposing it imagewise and then convert the latent image to a toner image by a powdery developer. When a developing device included in the apparatus is implemented with a two component type developer, i.e., a mixture of toner and carrier, it is supplemented with a toner from time to time since it sequentially consumes it. This is also true with a developing device of the type using a one component type developer, i.e., a toner.
It has been customary to provide the apparatus with a toner container storing a toner to be supplemented to the developing device and to supplement, when the amount of toner remaining in the container becomes short or zero, a toner from a toner cartridge or toner bottle to the container, as disclosed in Japanese Patent Application No. 280528/1988. However, this scheme has a problem that the capacity and, therefore, the amount of toner supplement available with a toner cartridge or toner bottle is limited. Specifically, when a great number of images are continuously formed, the operator has to supplement a toner to the toner container frequently, consuming time and labor.
In the light of this, Japanese Patent Laid-Open Publication No. 173683/1990, for example, discloses an image forming apparatus using another or second toner container in addition to the above-mentioned or first toner container. A toner stored in the second container is conveyed to the first container by, for example, a conveyor member and a tubing which guides the toner being transported by the conveyor member. When the amount of toner remaining in the first toner container becomes short or zero, the toner is supplemented from the first container to the second container. Likewise, when the amount of toner remaining in the second container becomes short or zero, a toner is supplemented to the second container. This approach frees the operator from the time- and labor-consuming supplementing work over a long period of time and thereby promotes easy operation of the apparatus. This is especially true when the second container is provided with a great capacity.
However, even the scheme using the first and second toner containers has a problem left unsolved, as follows. When the developing device consumes a particularly great amount of toner, the amount of toner to be supplied from the first toner container to the developing device increases too sharply for the supplement from the second container to the first container to follow. Then, it is likely that the first container runs out of toner despite that the second container is filled with the toner. Although the image forming operation of the apparatus may be interrupted for a moment and then resumed after the supplement of a predetermined amount of toner from the second container to the first container, this would lower the image forming efficiency.
To eliminate the above problem, an arrangement may be made such that the ability to convey the toner from the second container to the first container is enhanced so as to feed, when the absence of toner in the first container is sensed, a great amount of toner from the second container to the first container at a time. This, however, brings about another problem that the great amount of toner is conveyed while suddenly filling up the tubing of the conveying means and, therefore, apt to block up the tubing. Should the conveying means be so constructed as to transport a great amount of toner, the structure would be scaled up to increase the cost. Moreover, when the toner consumption by the developing device decreases after the increase, the amount of toner being supplemented to the first container will become excessive, blocking up the first container.
While the second container provided in addition to the first container reduces the number of times that the operator or serviceman has to supplement a toner, not all of the users need the second container which adds to the cost. Preferably, therefore, the second container should be put on the market as an optional unit, so that a user who does not need it may purchase the image forming apparatus only. In the image forming apparatus without the second container, a toner is supplied to the developing device from the first container only. When the amount of toner remaining in the first container becomes short or zero, the operator supplements a toner to the container from, for example, a toner cartridge.
On the other hand, in the case of the image forming apparatus with the second container, the operator supplements a toner to the second container. The apparatus is so constructed as to allow the operator to supplement a toner contained in, for example, a toner cartridge even to the first container for thereby serving the needs of users who do not want the second container. The operator, therefore, is apt to supplement a toner to the first container by accident. Then, an excessive amount of toner would be filled in the first container to block it up or to exert an excessive load on an agitator disposed in the container for agitating the toner and, in the worst case, damage the agitator. In addition, even when the operator is expected to supplement a toner to the second container, the condition which allows a toner to be supplemented to the first container as well would confuse the operator.